REGULATION OF CILIARY MOTILITY IN PARAMECIUM BY CAMP AND CGMP

Paramecium tetraurelia is a useful model system for the study of signa l transduction mechanisms that couple changes in membrane potential to changes in ciliary motility, and cAMP and cGMP have been implicated a s second messengers in this system. Changes in ciliary beat are correl ated with changes in the membrane potential, V-m, with hyperpolarizati ons resulting in increased beat frequency and increased forward swimmi ng speed. The intercellular levels of cAMP and cGMP vary with V-m, and increasing internal cAMP by microinjection or the use of membrane per meant analogs results in increased swimming speed. In addition, cAMP a nd cGMP modify the swimming behavior of permeabilized Paramecium, and the sensitivity of cilia to cAMP and cGMP depends upon the location of the cilia on the cell surface. However, increasing internal cAMP also results in hyperpolarization and increasing internal cAMP does not ov erride the effect of V-m. These results have lead to two conflicting m odels to describe the role of cAMP: (1) stimulus --> hyperpolarization of V-m --> increased [cAMP](in) --> increased beat frequency, and (2) stimulus --> increased [cAMP](in) --> hyperpolarization of V-m --> in creased beat frequency. This review discusses the data that support th ese models and possible experimental approaches to resolve the paradox presented by these models.